In sputtering systems of the general type herein described, a high vacuum gas is ionized between two electrodes, and a target of the material to be sputtered is located in the ionized region where ions bombard the target and dislodge atomic size particles. A substrate or work-piece is positioned to have the sputtered material deposited on its surface. For sputtering a conductive material, the target can be constructed to function as the cathode electrode. For sputtering a dielectric material such as silicon dioxide, silicon nitride, glass, quartz and the like, the target can be mounted on the cathode electrode. The substrates are usually positioned on the anode electrode. In the application of interest herein, sputtering is illustrated in the formation of thin coatings on semiconductor substrates for the fabrication of integrated circuit devices.
Typical timed RF sputter deposition systems are described in U.S. Pat. No. 4,131,533 issued Dec. 26, 1978 to J. A. Bialko et al, and in the article by J. S. Logan entitled "Control of RF Sputtered Film Properties Through Substrate Tuning", IBM Journal of Research and Development, Volume 147, pages 172-175, 1970.
RF sputtering with radio frequency in the low Megahertz range, for example from about 13 MHz up to around 40 MHz, is a typical range that is employed in the art. With properly selected frequency and applied voltage, the sputtering action can be confined to the target, e.g. dielectric, alone or the sputtering action may also occur from the substrates disposed on the anode, whereby atomic particles are "resputtered" from the substrates, similar to the sputtering which occurs from the target material comprising the cathode.
In any sputtering operation, it is desirable that only target material or, in the case of resputtering (as for sputter etching) substrate material be ejected. Therefore, it has been found necessary to protect other parts or elements of the sputtering system associated with the cathode and anode from the bombarding action of the ions. This can be accomplished by placing shields, normally grounded, around the cathode or anode or both. However, in RF sputtering, such shielding is not without penalties since such shields can divert current from the electrodes.
Operation of tuned substrate coating or deposition systems with high resputtering levels brings into play the so-called "flip point" or point of system instability which occurs at maximum resputtering. In the case of some 13 MHz systems, it can be estimated from voltage measurements, that planar mode operation (e.g. planar coatings on substrates), is achieved at 56% of maximum resputtering, as compared to 40 MHz systems which must operate at 85% of maximum resputtering. This means that the 40 MHz systems have a much narrower margin of safety for normal deviations.
Also, in conventional sputtering systems, the plasma is sensitive to mechanical set-up tolerances and background pressure variations which manifest themselves by arcing. Minor arcs will leave a light brown spot on a cathode shield ring. Major arcs will leave a dark blue or black scorched mark on the cathode shield ring and the chamber walls. In some cases the arc is visible. In such a case, a small fire-ball emitting intense bluish white light will track along the chamber wall until the RF power is removed from the electrodes. The arcs can also melt the surface of the metal chamber, which melted particles will spray all over in the chamber, causing potential product yield reduction. As is obvious, this is an unacceptable condition in sputtering systems wherein an insulating layer is deposited over thin metal conductors on a surface of semiconductor devices. Accordingly, an RF sputtering system is desired with reduced arcing, increased stability and reduced flaking contamination.